Since the introduction of microwave ovens, it has been recognized that the spatial distribution of the microwave energy in the cavity tends to be nonuniform. Because this nonuniformity may cause the undesirable condition of hot and cold spots within food being cooked, there has been extensive and continuous development work to improve the time averaged spatial distribution of energy. The spatial distribution is partially a function of reflections of microwave energy off of the conductive cavity walls thereby producing complex configurations of electromagnetic fields commonly referred to as modes. Simply stated, a major reason for the nonuniformity of the spatial distribution of microwave energy is the constructive and destructive interference of reflections. For example, where reflections add, a hot spot is created and where they subtract, a cold spot is created.
The principle approach for improving nonuniform heating in prior art microwave ovens has been to use a mode stirrer which attempts to randomize reflections and alter the modes by introducing a time varying scattering of the microwave energy. Typically, a mode stirrer is a metal paddle or propeller which rotates adjacent to the junction of the waveguide and the oven cavity. The object is to alter the modes so that the spatial positions of constructive and destructive interference move.
Another approach for improving uniformity has been to use a turntable within the microwave cavity to rotate the food and thereby move it through the hot and cold spots.
Another approach such as described in U.S. Pat. No. 4,414,453 utilizes a primary radiator that radiates microwave energy downward to the food in a directive pattern characteristic of antenna design. Rotation of the antenna sweeps the pattern over the food to improve the uniformity on a time average basis. Ideally, the rotation would result in concentric circles of uniform doneness. Radial evenness can be adjusted by changing the design of the pattern of the primary radiator. Further improvement is attained by increasing the diversity of the field patterns produced. One way of doing this has been to offset the radiating source from the center of rotation. Even though the original object of the primary radiator was to provide a somewhat directive pattern whereby a substantial portion of the microwave energy would be absorbed by the food before reflecting off the walls, a significant part of the radiation was still reflected before absorption. The reflections still interfered with the directly radiated wave within the food to increase or decrease the intensity depending on the difference of the phase of the interfering wave.
U.S. Pat. No. 4,336,434, issued June 22, 1982, states that a circularly polarized pattern provides more uniform spatial distribution of energy than linearly polarized. The explanation given therein is that this results because circularly polarized energy is reflected with a reversed direction of rotation. The circularly polarized energy is described as being provided by either using a waveguide that propagates circular polarization or using X slot apertures. Phase shifters are described between X slot apertures in order to provide beam steering for improving uniformity.